19. Genetics, Behavior and Aging. The epsilon 4 allele of apolipoprotein E (ApoE-e4) has been identified as a genetic risk factor for cognitive impairment, cognitive decline, and the experience of Alzheimer's Disease (AD). However, the presence of ApoE-e4 does not guarantee these cognitive outcomes. Therefore, one important direction for future research is to identify variables that may interact with ApoE genotype to determine cognitive outcomes. Physical activity is a behavioral variable that may play a role in the expression of the ApoE genotype relative to cognition. Physical activity has been shown to be associated with better cognitive abilities and with lessened cognitive decline with age. Further, the effects of physical activity on cognition have been shown in men to be moderated by ApoE genotype. Thus, the primary purpose of the proposed study is to add to the extant literature by testing the interactive effects of physical activity and ApoE genotype on the cognitive performance of a sample of older women. The literature will also be extended by assessing aerobic fitness which will provide insight into the causal mechanisms which may underlie the relationships being tested. Community-dwelling older women (50-80 years) who have a family history of AD will be recruited. Participants will perform a battery of cognitive tests, will have their blood drawn to determine their ApoE genotype, will complete a physical activity questionnaire, and will perform a maximal aerobic fitness test. It is hypothesized that the beneficial effects of physical activity and aerobic fitness on cognitive performance will be moderated by the presence of the ApoE-4 allele. Regression analyses will be used to test these effects. This study will advance our knowledge about the behavioral variables that modify genetic risk for cognitive impairment in older women. ApoE-4 is well established as a risk factor for cognitive decline and for the experience of AD. However, behavioral variables such as physical activity may impact the expression of this genetic risk factor and this study will extend our knowledge regarding this relationship. Additionally, the findings of this study will have important implications for identifying participants for whom physical activity may be especially beneficial with regards to cognition.